A power amplifier used for a wireless communication system and the like needs to have high linearity and high efficiency for input/output characteristics. On the other hand, a recent wireless communication system frequently handles a signal in which the average value of signal amplitudes and the maximum amplitude are different to a large extent, due to use of a multi-value digital modulation system or the like. When such a signal is amplified, an operation point of a transistor is set to be able to amplify the signal up to the maximum amplitude without distortion in a conventional power amplifier. Therefore, in the power amplifier, there remains substantially no period of time for the transistor that operates with relatively high efficiency to operate around a saturation output level, resulting in a decrease in efficiency.
To solve this problem, conventionally, techniques for increasing an efficiency of a power amplifier with maintaining linearity have been studied. As one of the techniques thereof, a Doherty amplifier is available.
FIG. 1 is a block diagram illustrating a configuration example of a Doherty amplifier.
As illustrated in FIG. 1, a Doherty amplifier 1 includes a main amplifier 2 that always carries out an amplification operation of a signal, a peak amplifier 3 that operates during high power output, a divider 4 that divides an input signal to the main amplifier 2 and the peak amplifier 3, and a combiner 5 that combines an output signal of the main amplifier 2 and an output signal of the peak amplifier 3 and outputs the combined signal.
For the main amplifier 2, there is used an amplifying circuit including one transistor or a push-pull amplifying circuit including two transistors, and the amplifying circuit is usually biased so as to operate in AB-class or B-class mode.
For the peak amplifier 3, there is used an amplifying circuit including one transistor or a push-pull amplifying circuit including two transistors, and the amplifying circuit is usually biased so as to operate in C-class mode.
The divider 4 equally divides power of an input signal to the main amplifier 2 and the peak amplifier 3 and delays a phase of the signal supplied to the peak amplifier 3 by 90 degrees with respect to a phase of the signal supplied to the main amplifier 2. For the divider 4, for example, a well-known 3-dB coupler is used. The 3-dB coupler includes four input/output terminals, an RF signal is input from one input terminal, and the other input terminal is grounded via a terminating resistance. From one output terminal of the 3-dB coupler, a signal having the same phase as the input signal is output and supplied to the main amplifier 2. From the other output terminal of the 3-dB coupler, a signal delayed by 90 degrees with respect to the input signal is output and supplied to the peak amplifier 3.
For the combiner 5, for example, a well-known quarter wavelength transmission line (quarter wavelength line) is used so that an output signal of the main amplifier 2 and an output signal of the peak amplifier 3 are combined in phase. The quarter wavelength line is realized with a print pattern formed on a printed circuit board. The Doherty amplifier 1 is described also in, for example, PTL 1.
When a power amplifier that outputs large power is realized using the Doherty amplifier 1 illustrated in FIG. 1, a plurality of Doherty amplifiers 1 are generally provided, the same signal is divided to each Doherty amplifier 1, and output signals of each Doherty amplifier 1 are combined to be output. Such a power amplifier that outputs large power is being considered to be used as a power amplifier for, for example, a television transmitter that transmits television broadcast signals. FIG. 2 illustrates a configuration example of a power amplifier of the background technique including the plurality of Doherty amplifiers 1.
FIG. 2 is a block diagram illustrating a configuration example of the power amplifier of the background technique.
As illustrated in FIG. 2, a power amplifier 10 of the background technique includes an RF signal input port 11, a variable attenuator 12, a variable phase shifter 13, a driver stage 14, an n (n is a positive integer equal to or greater than 2) divider 15, a final stage 16, an n combiner 17, and an RF signal output port 18.
FIG. 2 illustrates an example in which the driver stage 14 includes one Doherty amplifier 1 and the final stage 16 includes n Doherty amplifiers 1 connected in parallel. As described above, a main amplifier of each Doherty amplifier 1 included in the driver stage 14 and the final stage 16 is biased so as to operate in AB-class or B-class mode, and a peak amplifier thereof is biased so as to operate in C-class mode.
The variable attenuator 12 is configured to be settable for a desired attenuation amount by a control signal supplied from outside and attenuates an RF signal input from the RF signal input port 11 and outputs the attenuated RF signal.
The variable phase shifter 13 is configured to be settable for a desired phase shift amount by a control signal supplied from outside and delays a phase of the RF signal output from the variable attenuator 12 and outputs the resulting RF signal.
The driver stage 14 amplifies the RF signal output from the variable phase shifter 13 and outputs the amplified RF signal to the n divider 15.
The n divider 15 divides the RF signal supplied from the driver stage 14 to the n Doherty amplifiers 1 included in the final stage 16.
The final stage 16 amplifies the RF signals divided by the n divider 15 using the n Doherty amplifiers 1, respectively.
The n combiner 16 combines the RF signals amplified by each Doherty amplifier 1 included in the final stage 16 and outputs the combined RF signal from the RF signal output port 18.
In the Doherty amplifier 1 illustrated in FIG. 1, when the peak amplifier 3 does not operate, in other words, when a level of an input signal is low, an impedance of the output terminal of the peak amplifier 3 viewed from the output terminal of the main amplifier 2 needs to be open. When the impedance of the output terminal of the peak amplifier 3 is not open, a part of an output power of the main amplifier 2 sneaks to the peak amplifier 3 and then an output power of the combiner 5 decreases only by a portion thereof, resulting in occurrence of a power loss. The quarter wavelength line configuring the combiner 5 is also used to cause the impedance of the output terminal of the peak amplifier 3 viewed from the output terminal of the main amplifier 2 to be open. However, the quarter wavelength line is produced by determining a substrate material and a physical line length in accordance with a used frequency band so that characteristics thereof strongly depends on the frequency, resulting in a problem in which a usable frequency range is limited.
On the other hand, for example, a television broadcast signal has a wide frequency range used as a carrier (170-230 MHz for the VHF band and 470-862 MHz for the UHF band) so that, when the power amplifier 10 including a plurality of Doherty amplifiers illustrated in FIG. 2 is used for a television transmitter, it is difficult for the power amplifier 10 to cover the entire frequency band of the VHS band or the UHF band.
As a countermeasure for such a problem, it is conceivable that there is a configuration in which, for example, the VHF band or the UHF band is divided into a plurality of frequency bands and the power amplifier 10 dedicated for each frequency band is provided.
However, for such a configuration, components (the divider 4, the main amplifier 2, the peak amplifier 3, the combiner 5, and the like) different for each frequency band need to be produced, resulting in an increase in production cost. Further, it is difficult to start production until determining, for example, how the entire frequency band to be amplified is divided, resulting in a decrease in the production efficiency of the power amplifier 10.
A television broadcast is an important social infrastructure so that pieces of backup equipment are usually provided so as to prevent a sudden stop of broadcasting due to an equipment failure. Therefore, in a configuration in which the power amplifier 10 dedicated for each of a plurality of frequency bands is provided, a user managing a television transmitter prepares backup components for each of the plurality of frequency bands. Accordingly, a purchasing expense regarding the television transmitter by the user increases and also the maintenance of each power amplifier 10 is cumbersome.
In recent years, transistors operating even at a higher frequency have been developed, and the main amplifier 2 and the peak amplifier 3 capable of covering the entire band even in the VHF band or the UHF band have been realizable. Further, regarding the divider 4, the divider 4 operating with a required performance in a relatively wide frequency band has been realizable, similarly to a well-known Wilkinson power divider circuit.
However, regarding the quarter wavelength line used in the combiner 5 of the Doherty amplifier 1 (hereinafter, referred to as the “Doherty combiner”), a physical line length is determined in accordance with a substrate material and a frequency band so that when, for example, a frequency band to be used is changed, a printed circuit board or the like where a quarter wavelength line is formed needs to be exchanged. Alternatively, processing for the printed circuit board such as cutting or extending of the quarter wavelength line is necessary.
Accordingly, in a configuration in which a plurality of Doherty amplifiers is provided to realize a power amplifier that outputs large power and such a power amplifier is prepared for each of a plurality of frequency bands, it is necessary to exchange or process a large number of printed circuit boards each time a frequency band to be used is changed, resulting in very cumbersome operations.